BLTouch is an auto-leveling sensor that uses a retractable z-stop instead of a inductive sensor. The indiegogo campaign ended in November and they're selling them direct now. I've ordered a couple to test. Precision is an initial concern but the idea is interesting.

I finally got the BLTouch working. It was a bit of a learning curve for me, but now that I know what I'm doing, I could probably do it in an hour or so. The steps will vary somewhat depending on your particular M2 revision. Mine was purchased in 06/2015, and was upgraded to a V4 extruder. It has the old silver and black Z motor. This revision had the Z min endstop (on top of the printer), and not the Z max endstop (like the newer revision). I do not have an LCD. Depending on your accessories/modifications/etc, you may have to plug the BLTouch into different pins on the RAMBo.

You will need:
- BLTouch Probe
- Long servo wire - about 3ft with three wires - I like the twisted 22AWG wire.
- Dupont connectors ("Universal"/JR Servo connectors).
- Dupont connector crimp tool. You can try needle-nose pliers, but a good crimper does a much better job.
- Basic knowledge of 3D printer firmware. Check out Thomas Sanladerer's Firmware Basics Guide on YouTube.

Step 3: Remove the fans and old filament guide. Bolt the BLTouch to the new filament guide. Feed both BLTouch wires through the rectangular hole and replace your old filament guide.
I had to enlarge the rectangular hole to get the wires to feed through - would have been easier to temporarily remove the dupont connectors.

Step 4: If you don't already have a modified firmware, go here, and determine the correct firmware version and download it: http://makergear.wikidot.com/m2-firmware
Make sure you're 100% certain you have the correct firmware. Download and unzip the file somewhere on your computer.

Step 5: Get the correct version of the Arduino IDE for editing the firmware. The website above lists a specific version - trying to use a more recent version didn't work for me.

Step 6: Remove the Z min endstop. Cut the wires close to the endstop. You won't need the wire to the LED - I removed it entirely.
You'll be left with a black and white wire. Strip the two wires and crimp on a male 2-pin dupont connector. (YouTube has great tutorials on dupont connectors.)
I had to re-route the Z min endstop wire from the RAMBo and bundle it with the wires to the extruder. These plug right into the black and white wires on the BLTouch.
These two wires enable the BLTouch to act as your new Z min stop. You should not have to relocate these wires on the RAMBo itself but you may need to extend the wires.
Leave them connected to the same pins in the RAMBo as the original Z min endstop.

Step 6: You'll need a new, long 3-wire servo wire (brown/orange/yellow, if you want to match the probe). Crimp a 3-pin male connector to one end, and a female to the other.
Plug the male end into the BLTouch and route the wire to the RAMBo with the other extruder/fan wires. The female end will get plugged into the "MOTOR EXT" pins.
Plug the red wire into the MX1 "+" pin.
Plug the brown wire into the MX1 "-" pin.
Plug the yellow wire into the MX1 "signal" pin, just to the left of the "-" pin.
These three wires enable the BLTouch to act as a servo, raising and lowering the probe.
All three pins should be next to each other in a row. The signal pin is #22.
Note that your negative lead is in the middle of the other two pins (unlike the end connected to the BLTouch).

Step 8: Turn your printer on. The BLTouch probe should move in and out a couple of times. If this doesn't happen, something went wrong with your wiring.

Step 9: Open the Arduino IDE, and edit your Marlin.ino file (in the new firmware you just downloaded). Navigate to the Configuration.h sketch.

Find this line and ensure it is set to false:
const bool Z_MIN_ENDSTOP_INVERTING = false;

Uncomment this line:
#define ENABLE_AUTO_BED_LEVELING

Update the lines below with these numbers: (You can play with these to fine-tune.)
// set the rectangle in which to probe
#define LEFT_PROBE_BED_POSITION 20
#define RIGHT_PROBE_BED_POSITION 175
#define BACK_PROBE_BED_POSITION 230
#define FRONT_PROBE_BED_POSITION 20

Update your probe offsets:
// These settings assume you're using the same filament guide as I am.
#define X_PROBE_OFFSET_FROM_EXTRUDER 17
#define Y_PROBE_OFFSET_FROM_EXTRUDER 0
#define Z_PROBE_OFFSET_FROM_EXTRUDER -2.05 // You may need to play with this to get the right "squish" on your first layer.